Charged-particle-beam ("CPB") pattern-transfer apparatus have been developed for transferring patterns from a mask to a sensitized substrate, typically a semiconductor wafer that is coated with a thin layer of a suitable resist. One such apparatus using an electron beam is described in Japanese Kokai patent document no. Hei 5-160012. This apparatus illuminates a mask with an electron beam and then forms a demagnified image of patterns on the mask on a sensitized wafer using a two-stage projection lens.
Electron-beam optical systems exhibit unacceptable levels of aberrations when used to illuminate large fields. Therefore, a pattern from an entire mask cannot be transferred to the wafer in a single exposure using such an optical system. Instead, the mask is divided into a plurality of small "subfields" and the pattern portion defined by each subfield is sequentially transferred to the wafer. One system that transfers subfields in this manner is described in U.S. Pat. No. 5,260,151, incorporated herein by reference.
In CPB pattern-transfer apparatus in which the mask is divided into subfields, the lens downstream of the mask generally has a focal length that is approximately equal to the focal length of a condenser lens that focuses the CPB on the subfield. If these two focal lengths are approximately equal, then spherical aberration is reduced. In addition, a simple lens design can be used for both lenses. However, if these two lenses have approximately equal focal lengths, then the distance from the CPB source (e.g., an electron gun) to the wafer tends to be long and the CPB pattern-transfer apparatus tends to be correspondingly large. In some cases, the apparatus is too large to fit inside an ordinary clean room.
These apparatus generally limit the mask area illuminated by the CPB to a selected subfield by trimming the CPB using an aperture to appropriately limit the transverse area of the CPB. The aperture defines an illumination region that is projected onto the mask with a magnification greater than unity. Because an enlarged image of the aperture is projected onto the selected subfield, the aperture must be small. The manufacture of an aperture plate defining a small aperture is difficult. In addition, the edges of the aperture plate receive intense illumination by the CPB causing large temperature variations. These large temperature variations cause unacceptable thermal variations in the dimensions of the aperture.
In addition, in conventional CPB pattern-transfer apparatus, the CPB propagates through the optical system at different angles for on-axis and off-axis mask subfields. This leads to increased aberrations that are especially serious for off-axis subfields.